Perfusion imaging by a flow-sensitive alternating inversion recovery (FAIR) technique: application to functional brain imaging

A primer on functional magnetic resonance imaging

In this manuscript, basic principles of functional magnetic resonance imaging (fMRI) are reviewed. In the first section, two intrinsic mechanisms of magnetic resonance image contrast related to the longitudinal and transverse components of relaxing spins and their relaxation rates, T(1) and T(2), are described. In the second section, the biophysical mechanisms that alter the apparent transverse relaxation time, T(2*), in blood oxygenation level dependent (BOLD) studies and the creation of BOLD activation maps are discussed. The physiological complexity of the BOLD signal is emphasized. In the third section, arterial spin labeling (ASL) measures of cerebral blood flow are presented. Arterial spin labeling inverts or saturates the magnetization of flowing spins to measure the rate of delivery of blood to capillaries. In the fourth section, calibrated fMRI, which uses BOLD and ASL to infer alterations of oxygen utilization during behavioral activation, is reviewed. The discussion concludes with challenges confronting studies of individual cases.

FAIR-TrueFISP imaging of cerebral perfusion in areas of high magnetic susceptibility differences at 1.5 and 3 Tesla

PURPOSE

To estimate cerebral blood perfusion in areas of strong magnetic susceptibility changes with high spatial and temporal resolution using a flow-sensitive alternating inversion recovery (FAIR) arterial spin labeling (ASL) method.

MATERIALS AND METHODS

We implemented an ASL method that is capable of imaging perfusion in areas of high magnetic susceptibility changes by combining a FAIR spin preparation with a true fast imaging in steady precession (TrueFISP) data acquisition strategy. A TrueFISP readout sequence was applied especially in regions with magnetic field inhomogeneities and compared with corresponding FAIR measurements obtained with a standard echo-planar imaging (EPI) readout. Quantitative perfusion images were obtained at 1.5 Tesla (1.5T) from eight healthy volunteers (24-42 years old) and one patient (23 years old). FAIR-TrueFISP perfusion images were compared with FAIR echo-planar images. T1 maps, which are necessary for quantitative perfusion estimation, were obtained with inversion recovery (IR) TrueFISP and IR EPI. Additionally, high-resolution perfusion measurements were performed on four volunteers at 3T.

RESULTS

The two ASL perfusion imaging modalities yielded comparable diagnostic image quality in brain areas with low susceptibility differences at 1.5T. Cerebral perfusion of gray matter (GM) areas was 105.7 +/- 5.2 mL/100 g/minute for FAIR-TrueFISP and 88.8 +/- 14.6 mL/100 g/minute for FAIR-EPI at 1.5T, and 70.4 +/- 7.1 mL/100 g/minute for FAIR-TrueFISP and 63.5 +/- 6.9 mL/100 g/minute for FAIR-EPI at 3.0T. Higher perfusion values at 1.5T were due to more pronounced partial-volume effects from fast moving spins at lower spatial resolution. The FAIR-TrueFISP sequence showed no significant distortions and markedly reduced signal void artifacts in areas of high susceptibility changes (e.g., near brain-bone transitions and close to metallic clips) compared to FAIR-EPI. At 3T, highly resolved FAIR-TrueFISP perfusion images were acquired with an in-plane resolution of up to 1.3 mm.

CONCLUSION

FAIR-TrueFISP allows for assessment of cerebral perfusion in areas of critically high susceptibility changes with conventional ASL methods.

Quantification of rodent cerebral blood flow (CBF) in normal and high flow states using pulsed arterial spin labeling magnetic resonance imaging

PURPOSE

To implement a pulsed arterial spin labeling (ASL) technique in rats that accounts for cerebral blood flow (CBF) quantification errors due to arterial transit times (dt)-the time that tagged blood takes to reach the imaging slice-and outflow of the tag.

MATERIALS AND METHODS

Wistar rats were subjected to air or 5% CO(2), and flow-sensitive alternating inversion-recovery (FAIR) perfusion images were acquired. For CBF calculation, we applied the double-subtraction strategy (Buxton et al., Magn Reson Med 1998;40:383-396), in which data collected at two inversion times (TIs) are combined.

RESULTS

The ASL signal fell off more rapidly than expected from TI = one second onward, due to outflow effects. Inversion times for CBF calculation were therefore chosen to be larger than the longest transit times, but short enough to avoid systematic errors caused by outflow of tagged blood. Using our method, we observed a marked regional variability in CBF and dt, and a region dependent response to hypercapnia.

CONCLUSION

Even when flow is accelerated, CBF can be accurately determined using pulsed ASL, as long as dt and outflow of the tag are accounted for.

Simultaneous laser Doppler flowmetry and arterial spin labeling MRI for measurement of functional perfusion changes in the cortex

This study compares laser Doppler flowmetry (LDF) and arterial spin labeling (ASL) for the measurement of functional changes in cerebral blood flow (CBF). The two methods were applied concurrently in a paradigm of electrical whisker stimulation in the anaesthetised rat. Multi-channel LDF was used, with each channel corresponding to different fiber separation (and thus measurement depth). Continuous ASL was applied using separate imaging and labeling coils at 3 T. Careful experimental set up ensured that both techniques recorded from spatially concordant regions of the barrel cortex, where functional responses were maximal. Strong correlations were demonstrated between CBF changes measured by each LDF channel and ASL in terms of maximum response magnitude and response time-course within a 6-s-long temporal resolution imposed by ASL. Quantitatively, the measurements of the most superficial LDF channels agreed strongly with those of ASL, whereas the deeper LDF channels underestimated consistently the ASL measurement. It was thus confirmed that LDF quantifies CBF changes consistently at a superficial level, and for this case the two methods provided concordant measures of functional CBF changes, despite their essentially different physical principles and spatiotemporal characteristics.

Velocity-selective arterial spin labeling

In pathologies in which slow or collateral flow conditions may exist, conventional arterial spin labeling (ASL) methods that apply magnetic tags based on the location of arterial spins may not provide robust measures of cerebral blood flow (CBF), as the transit delay for the delivery of blood to target tissues may far exceed the relaxation time of the tag. Here we describe current methods for ASL with velocity-selective (VS) tags (termed VSASL) that do not require spatial selectivity and can thus provide quantitative measures of CBF under slow and collateral flow conditions. The implementation of a robust multislice VSASL technique is described in detail, and data obtained with this technique are compared with those obtained with conventional pulsed ASL (PASL). The technical considerations described here include the design of VS pulses, background suppression, anisotropy with respect to velocity-encoding directions, and CBF quantitation issues.

Temporal lobe perfusion in the deaf: MR measurement with pulsed arterial spin labeling (FAIR)

RATIONALE AND OBJECTIVES

Experimental studies in animals have shown that loss of a primary sensory modality early in life may result in substantial alterations in cortical organization. This study was performed to measure cerebral perfusion in auditory cortex in congenitally deaf adults using the FAIR (Flow-sensitive Alternating Inversion Recovery) magnetic resonance imaging technique. Our hypothesis was that there would be relatively intact perfusion in auditory cortex.

MATERIALS AND METHODS

Twenty-six profoundly congenitally deaf subjects were compared with 15 control subjects. A FAIR perfusion slice was scanned through the superior temporal gyrus parallel to the Sylvian fissure while subjects were at rest. Perfusion maps were calculated and regions of interest were drawn over the superior temporal gyrus including auditory cortex and the medial occipital lobe.

RESULTS

The relative perfusion of the superior temporal gyrus (STG) was slightly less in the deaf (right STG = 0 .79 +/- 0.16, left = 0.93 +/- 0.29) compared with the hearing (right STG = 0.90 +/- 0.14, left = 0.98 +/- 0.31) when normalized to the occipital cortex, but the differences were not statistically significant. Both showed moderate left lateralization; however, only in the deaf did this reach statistical significance (P < .01).

CONCLUSIONS

In the resting state, the deaf demonstrate a relatively normal perfusion in the region of cortex usually associated with auditory function. Although the presumed underlying electrical activity may represent some degree of residual auditory function, it is likely that the normal level of perfusion reflects cortical reorganization and the early migration of nonauditory processing into this area.

Accelerated parallel imaging for functional imaging of the human brain

Accelerated parallel imaging (PI) techniques have recently been applied to functional imaging experiments of the human brain in order to improve the performance of commonly used single-shot techniques like echo-planar imaging (EPI). Potential benefits of PI-fMRI include the reduction of geometrical distortions due to off-resonance signals, the reduction of signal-loss in areas with substantial signal inhomogeneity, increases of the spatial and temporal resolution of the fMRI experiment and reduction of gradient acoustic noise. Although PI generally leads to a substantial decrease in image signal-to-noise ratio (SNR), its effect on the temporal stability of the signal, which ultimately determines fMRI performance, is only partially determined by image SNR. Therefore, the penalty for using PI is generally not as severe as the SNR reduction. The majority of problems related to single-shot techniques become more severe at an increased magnetic field strength, making PI an important tool in achieving the full potential of fMRI at high field.

Quantifying CBF with arterial spin labeling

The basic principles of measuring cerebral blood flow (CBF) using arterial spin labeling (ASL) are reviewed. The measurement is modeled by treating the ASL method as a magnetic resonance imaging (MRI) version of a microsphere study, rather than a diffusible tracer study. This approach, particularly when applied to pulsed ASL (PASL) experiments, clarifies that absolute calibration of CBF primarily depends on global properties of blood, rather than local tissue properties such as the water partition coefficient or relaxation time. However, transit delays from the tagging region to the image voxel are a potential problem in all standard ASL methods. The key to quantitative CBF measurements that compensate for this systematic error is to create a well-defined bolus of tagged blood and to ensure that all of the bolus has been delivered to an imaging voxel at the time of measurement. Two practical technical factors considered here are 1) producing a tagged bolus with a well-defined temporal width and 2) accounting for reduction in magnitude of the tagged magnetization due to relaxation. The ASL approach has the potential to provide a robust estimation of CBF, although the timing of water exchange into tissue and the effects of pulsatile flow require further investigation.

Quantifying CBF with pulsed ASL: technical and pulse sequence factors

We summarize here current methods for the quantification of CBF using pulsed arterial spin labeling (ASL) methods. Several technical issues related to CBF quantitation are described briefly, including transit delay, signal from larger arteries, radio frequency (RF) slice profiles, magnetization transfer, tagging efficiency, and tagging geometry. Many pulsed tagging schemes have been devised, which differ in the type of tag or control pulses, and which have various advantages and disadvantages for quantitation. Several other modifications are also available that can be implemented as modules in an ASL pulse sequence, such as varying the wash-in time to estimate the transit delay. Velocity-selective ASL (VS-ASL) uses a new type of pulse labeling in which inflowing arterial spins are tagged based on their velocity rather than their spatial location. In principle, this technique may allow ASL measurement of cerebral blood flow (CBF) that is insensitive to transit delays.

Model-free arterial spin labeling quantification approach for perfusion MRI

In this work a model-free arterial spin labeling (ASL) quantification approach for measuring cerebral blood flow (CBF) and arterial blood volume (aBV) is proposed. The method is based on the acquisition of a train of multiple images following the labeling scheme. Perfusion is obtained using deconvolution in a manner similar to that of dynamic susceptibility contrast (DSC) MRI. Local arterial input functions (AIFs) can be estimated by subtracting two perfusion-weighted images acquired with and without crusher gradients, respectively. Furthermore, by knowing the duration of the bolus of tagged arterial blood, one can estimate the aBV on a voxel-by-voxel basis. The maximum of the residue function obtained from the deconvolution of the tissue curve by the AIF is a measure of CBF after scaling by the locally estimated aBV. This method provides averaged gray matter (GM) perfusion values of 38 +/- 2 ml/min/100 g and aBV of 0.93% +/- 0.06%. The average CBF value is 10% smaller than that obtained on the same data set using the standard general kinetic model (42 +/- 2 ml/min/100 g). Monte Carlo simulations were performed to compare this new methodology with parametric fitting by the conventional model.

A comparison between blood oxygenation level-dependent and cerebral blood volume contrast in the rat cerebral and cerebellar somatosensoric cortex during electrical paw stimulation

PURPOSE

To implement and optimize cerebral blood volume (CBV)-weighted functional magnetic resonance imaging (fMRI) in the rat cerebral and cerebellar cortex during electrical paw stimulation.

MATERIALS AND METHODS

fMRI of the cerebral and cerebellar cortex was performed during electrical paw stimulation on a 7-T MRI system (MRRS, Guilford, UK) comparing the blood oxygenation level-dependent (BOLD) and CBV-weighted contrast with different ultrasmall particles of iron oxide (USPIO) contrast doses (NC100150, 30 mg Fe/mL; Amersham Health, Oslo, Norway) and different TE.

RESULTS

Doses of 15 and 20 mg/kg USPIO at TE = T*2 or TE = 14 msec almost doubled the contrast-to-noise ratio (CNR) of the activated areas in the cerebral cortex without affecting the overall signal-to-noise ratio (SNR) or the incidence of activation (100%). In the cerebellum the SNR decreased significantly with an increasing contrast dose. At a dose of 15 mg/kg, the CNR was slightly smaller than the CNR measured in the BOLD images, but the activation incidence seemed to be doubled. At 20 mg/kg, the CNR was slightly increased, but the activation incidence was lower. At both contrast doses the venous artifacts disappeared.

CONCLUSION

A USPIO contrast dose of 20 mg/kg proved to be beneficial for fMRI in the rat, even though it affected the CNR and SNR in the cerebral and the cerebellar cortex differentially.

Single-shot quantitative perfusion imaging of the human lung

The major drawback to quantitative perfusion imaging using arterial spin labeling (ASL) techniques is the need to acquire two images (tag and control), which must be subtracted in order to obtain a perfusion-weighted image. This can potentially result in misregistration artifacts, especially in lung imaging, due to varying lung inflation levels in different breath-holds. In this work a double inversion recovery (DIR) imaging technique that yields perfusion-weighted images of the human lung in a single shot is presented. This technique ensures the complete suppression of background tissue while it preserves signal from the blood. Furthermore, the perfusion-weighted images and an additional (independent) acquired reference scan can be used to obtain quantitative perfusion information from the lungs.

Spatially-confined arterial spin-labeling with FAIR

PURPOSE

To investigate the effectiveness of slab-selective inversion in pulsed arterial spin labeling with body coil excitation as a means to reduce large vessel contamination of the perfusion signal.

MATERIALS AND METHODS

Studies were conducted by varying the tagging width in multislice flow-sensitive alternating inversion recovery (FAIR) in conjunction with body coil excitation on a Siemens Sonata whole-body 1.5-T scanner. The results of spatially-confined tagging were then compared with conventional nonselective tagging in the presence and absence of a bipolar gradient crusher pair in order to determine the effectiveness of suppressing vascular signal and to estimate the bolus width that reaches the capillary bed.

RESULTS

It is shown in five volunteers, ages 23-38 years, that depending on the average velocity of the arterial blood flow in the tagging region, a bolus of 6-8 cm in width reaches the capillary bed at a fixed inversion time TI of 1.4 seconds, while a bolus of 11.2-16.5 cm in width enters the imaging region. Further, noticeable velocity differences have been found among the participating subjects, with averages ranging from 10.1 to 13.9 cm/second.

CONCLUSION

The data suggest that it is advantageous to replace nonselective global tagging in FAIR perfusion imaging with body coil excitation by spatially-confined tagging to reduce undesired residual tagged blood in large vessels.

Enduring representational plasticity after somatosensory stimulation

Somatosensory stimulation (SS), leading to increases in motor cortical excitability, influences motor performance in patients with brain lesions like stroke. The mechanisms by which SS modulates motor function are incompletely understood. Here, we used functional magnetic resonance imaging (fMRI, blood-oxygenation-level-dependent (BOLD), and perfusion imagings simultaneously acquired in a 3 T magnet) to assess the effects of SS on thumb-movement-related activation in three regions of interest (ROI) in the motor network: primary motor cortex (M1), primary somatosensory cortex (S1), and dorsal premotor cortex (PMd) in healthy volunteers. Scans were obtained in different sessions before and after 2-h electrical stimulation applied to the median nerve at the wrist (MNS), to the skin overlying the shoulder deltoid muscle (DMS), and in the absence of stimulation (NOSTIM) in a counterbalanced design. We found that baseline perfusion intensity was comparable within and across sessions. MNS but not DMS nor NOSTIM led to an increase in signal intensity and number of voxels activated by performance of median nerve-innervated thumb movements in M1, S1, and PMd for up to 60 min. Task-related fMRI activation changes were most prominent in M1 followed by S1 and to a lesser extent in PMd. MNS elicited a displacement of the center of gravity for the thumb movement representation towards the other finger representations within S1. These results indicate that MNS leads to an expansion of the thumb representation towards other finger representations within S1, a form of plasticity that may underlie the influence of SS on motor cortical function, possibly supporting beneficial effects on motor control.

Reduced cerebral blood flow and N-acetyl aspartate in a murine model of cerebral malaria

Cerebral malaria is an important cause of morbidity and mortality in many parts of the world. It has been suggested that cerebral malaria is associated with reduced perfusion due to the blockage of blood vessels by parasitized erythrocytes; although, no quantitative validation of this has been done. We infected C57BL/6 mice with the ANKA strain of Plasmodium berghei and on day 6 of infection we investigated alterations in brain function using arterial spin labeling MRI and proton MRS. MR images did not demonstrate signs of damage. However, there was a significant reduction in cerebral blood flow (P<0.012) and the ratio of N-acetyl-aspartate (NAA) to creatine (Cr) (P<0.01) relative to non-infected mice. The NAA/Cr ratios were significantly correlated with cerebral perfusion (r=0.87) suggesting a relationship between impaired oxygen delivery and neuronal dysfunction. Pathological examination revealed accumulations of damaged axons providing a correlate for the decreased NAA/Cr ratio in infected mice. This murine model will permit non-invasive studies of neurologic function during malarial infection.

Resolution and reproducibility of BOLD and perfusion functional MRI at 3.0 Tesla

Visual and somatosensory activation studies were performed on normal subjects to compare the spatial discrimination and reproducibility between functional MRI (fMRI) methods based on blood oxygen level-dependent (BOLD) and perfusion contrast. To allow simultaneous measurement of BOLD and perfusion contrast, a dedicated MRI acquisition technique was developed. Repeated experiments of sensory stimulation of single digits of the right hand showed an average variability of activation amplitude of 25% for BOLD data, and a significantly lower variability of 21% for perfusion data. No significant difference in the variability of the locus of activity was observed between the BOLD and perfusion data. In somatotopy experiments, digits II and V were subjected to passive sensory stimulation. Both the BOLD and perfusion data showed substantial overlap in the activation patterns from the two digits. In a retinotopy study, two stimuli were alternated to excite different patches of V1. Again there was substantial overlap between the activation patterns from both stimuli, although the perfusion performed somewhat better than the BOLD method. Particularly for the visual studies, the overlap in activation patterns was more than expected based on the fine-scale retinotopic mapping of cortical activity, suggesting that both BOLD and perfusion contrast mechanisms contribute substantially to the point-spread function (PSF).

Continuous arterial spin labeling using a train of adiabatic inversion pulses

PURPOSE

To develop a simple and robust magnetic resonance imaging (MRI) pulse sequence for the quantitative measurement of blood flow in the brain and cerebral tumors that has practical implementation advantages over currently used continuous arterial spin labeling (CASL) schemes.

MATERIALS AND METHODS

Presented here is a single-coil protocol that uses a train of hyperbolic secant inversion pulses to produce continuous arterial spin inversion for perfusion weighting of fast spin echo images. Flow maps of normal rat brains and those containing a 9L gliosarcoma orthotopic tumor model conditions were acquired with and without carbogen.

RESULTS

The perfusion-weighted images have reduced magnetization transfer signal degradation as compared to the traditional single-coil CASL while avoiding the use of a more complex two-coil CASL technique. Blood flow measurements in tumor and normal brain tissue were consistent with those previously reported by other CASL techniques. Contralateral and normal brain showed increased blood flow with carbogen breathing, while tumor tissue lacked the same CO(2) reactivity.

CONCLUSION

This variation of the CASL technique is a quantitative, robust, and practical single-coil method for measuring blood flow. This CASL method does not require specialized radiofrequency coils or amplifiers that are not routinely used for anatomic imaging of the brain, therefore allowing these flow measurements to be easily incorporated into traditional rodent neuroimaging protocols.

Hypercapnic normalization of BOLD fMRI: comparison across field strengths and pulse sequences

The blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) signal response to neural stimulation is influenced by many factors that are unrelated to the stimulus. These factors are physiological, such as the resting venous cerebral blood volume (CBV(v)) and vessel size, as well as experimental, such as pulse sequence and static magnetic field strength (B(0)). Thus, it is difficult to compare task-induced fMRI signals across subjects, field strengths, and pulse sequences. This problem can be overcome by normalizing the neural activity-induced BOLD fMRI response by a global hypercapnia-induced BOLD signal. To demonstrate the effectiveness of the BOLD normalization approach, gradient-echo BOLD fMRI at 1.5, 4, and 7 T and spin-echo BOLD fMRI at 4 T were performed in human subjects. For neural stimulation, subjects performed sequential finger movements at 2 Hz, while for global stimulation, subjects breathed a 5% CO(2) gas mixture. Under all conditions, voxels containing primarily large veins and those containing primarily active tissue (i.e., capillaries and small veins) showed distinguishable behavior after hypercapnic normalization. This allowed functional activity to be more accurately localized and quantified based on changes in venous blood oxygenation alone. The normalized BOLD signal induced by the motor task was consistent across different magnetic fields and pulse sequences, and corresponded well with cerebral blood flow measurements. Our data suggest that the hypercapnic normalization approach can improve the spatial specificity and interpretation of BOLD signals, allowing comparison of BOLD signals across subjects, field strengths, and pulse sequences. A theoretical framework for this method is provided.

Perfusion- and BOLD-based fMRI in the study of a human pathological model for task-related flow reductions

In the present work, an arteriovenous malformation was taken as a pathological model for studying task-related flow decreases during a motor task. Combined Blood Oxygen Level Dependent (BOLD)-perfusion experiments were applied in order to evaluate the relative sensitivity of these techniques to task-related reductions in cerebral blood flow (CBF). Results shows that, by matching the sensitivity of the methods (which exhibit a different contrast-to-noise ratio) in the primary motor cortex, the spatial extent of the regions of decreased perfusion signal is larger than those of the BOLD signal reduction. The above finding suggests that perfusion imaging, that already represents a gold standard method in the detection of vascular phenomena, may estimate task-related flow decreases in a functional time-series better than BOLD.

Perfusion imaging using arterial spin labeling

Arterial spin labeling is a magnetic resonance method for the measurement of cerebral blood flow. In its simplest form, the perfusion contrast in the images gathered by this technique comes from the subtraction of two successively acquired images: one with, and one without, proximal labeling of arterial water spins after a small delay time. Over the last decade, the method has moved from the experimental laboratory to the clinical environment. Furthermore, numerous improvements, ranging from new pulse sequence implementations to extensive theoretical studies, have broadened its reach and extended its potential applications. In this review, the multiple facets of this powerful yet difficult technique are discussed. Different implementations are compared, the theoretical background is summarized, and potential applications of various implementations in research as well as in the daily clinical routine are proposed. Finally, a summary of the new developments and emerging techniques in this field is provided.

Decreased cerebral perfusion correlates with increased BOLD hyperoxia response in transgenic mouse models of sickle cell disease

Neurological complications such as stroke are known consequences of sickle cell disease (SCD). In order to improve methods for the evaluation of stroke risk in SCD, MRI was used to evaluate cerebrovascular function in transgenic mouse models of human SCD. It is hypothesized that oxygen-sensitive imaging in the brain will reveal areas of excess deoxygenation that are either at risk of or the result of vaso-occlusion. Arterial spin labeling (ASL) perfusion was performed in order to correlate BOLD results with microvascular cerebral blood flow. Upon comparison with control animals, there was a relative increase in BOLD hyperoxia response of 42-67% (P < 0.001) in the transgenic mice while cerebral blood flow during normoxia was reduced by 30-40% (P < 0.02). Hyperoxia caused cerebral blood flow to decrease in control mice, whereas blood flow increased in the sickle transgenic mice. These results indicate impairment in brain autoregulation in the sickle cell transgenic mice leading to increased cerebral deoxyhemoglobin. Increased deoxyhemoglobin coupled with reduced perfusion may further increase the risk of vaso-occlusion and stroke. This may reflect polymer reduction or reduced cell adhesion during hyperoxia. The MRI protocol is noninvasive and thus directly applicable to a clinical population.

The physiology and metabolism of neuronal activation: in vivo studies by NMR and other methods

In this article, a review is made of the current knowledge concerning the physiology and metabolism of neuronal activity, as provided by the application of NMR approaches in vivo. The evidence furnished by other functional spectroscopic and imaging techniques, such as PET and optical methods, are also discussed. In spite of considerable amounts of studies presented in the literature, several controversies concerning the mechanisms underlying brain function still remain, mainly due to the difficult assessment of the single vascular and metabolic dynamics which generally influence the functional signals. In this framework, methodological and technical improvements are required to provide new and reliable experimental elements, which can support or eventually modify the current models of activation.

High field human imaging

This review article examines the state of knowledge regarding human imaging using MRI at high main magnetic field strengths. The article starts with a summary of the technical issues associated with magnetic field strengths in the range of 3-8 T, including magnet characteristics and the properties of radiofrequency magnetic fields, with special reference to sensitivity, power deposition, and homogeneity. The published data on tissue-water relaxation times in the brain is tabulated and the implications for contrast and pulse sequence implementation is elucidated. The behavior of the major fast imaging sequences, fast low angle shot (FLASH), rapid acquisition with relaxation enhancement (RARE), and echo planar imaging (EPI), is examined in this context. A number of anatomical images from 3 T systems are presented as examples. Particular attention is given to various forms of vascular imaging, namely, time of flight angiography, venography, and arterial spin labeling. The most complex changes in contrast with main magnetic field strength are in activation studies utilizing the blood oxygen level dependent mechanism, which are examined in detail. Improvements in spatial specificity are emphasized, particularly in conjunction with spin-echo imaging. The article concludes with a discussion of the current status and the potential impact of technical developments such as parallel imaging.

Comparison of FAIR technique with different inversion times and post contrast dynamic perfusion MRI in chronic occlusive cerebrovascular disease

The purpose of this study was to examine the signal change occurring with different inversion times (TIs) of the flow-sensitive alternating inversion recovery (FAIR) technique and to compare with the perfusion image obtained with Gd-DTPA injection. The subjects were 11 patients with unilateral occlusive cerebrovascular disease. Two FAIR images with different TIs (800 ms and 1600 ms) were measured for each patient and dynamic perfusion MRI was performed to produce four kinds of parameter maps: mean transit time (MTT), time to peak (TTP), relative cerebral blood flow (rCBF) and relative cerebral blood volume (rCBV) maps. Asymmetry ratios (ARs) between the affected and contra-lateral vascular sides were measured in both FAIR images and the four dynamic parameter maps. The AR of the MTT map of the four parameters showed the highest correlation with that of the FAIR images, especially with that of TI = 1600 ms (r = 0.829), and the AR of the rCBV map revealed the worst correlation with the FAIR images. The AR of the FAIR image with TI = 800 ms was less correlated with that of MTT than that with TI = 1600 ms. These results suggested that the signal intensity of the FAIR image was influenced by flow transition time and the change in TI could be used to select the flow with a different transition time. Our study suggested that a longer TI in the FAIR technique might be more useful than a shorter TI for evaluating chronic occlusive cerebrovascular disease in the clinical setting.

Cytotoxic edema is independent of NMDA ion channel activation following middle cerebral artery occlusion (MCAO). An in vivo autoradiographic and MRI study

Massive glutamate release is an important factor leading to ionic imbalance after occlusive stroke, which in turn contributes to cytotoxic edema formation. Currently, measurements of cytotoxic edema using 'diffusion weighted' MRI, is being used in human stroke studies, as a 'surrogate' end point for neuroprotective drug trials, including studies with glutamate antagonists. However, it is not fully understood to what extent glutamate-mediated N-methyl-D-aspartate (NMDA) receptor activation is related to 'cytotoxic' edema formation, and thus, to what degree apparent diffusion coefficient (ADC) changes, assessed by magnetic resonance imaging with 'ACD mapping', represent NMDA receptor activation. To study this relationship, four cats underwent permanent middle cerebral artery occlusion (MCAO). Edema formation was investigated using MRI with 'ACD mapping', while NMDA receptor activation was simultaneously detected in the same animals, using radio labeled 125IodoMK-801, which binds only in activated and open NMDA channels. At 5 h post-occlusion, a large area of edema could be found with significantly lower ADC values in the core and penumbral area of the ischemic lesion when compared to contralateral values. On corresponding sections of the feline brains, increased 125I-MK-801 binding was found in the infarct penumbra. However, there was no significant topographical correlation between ADC values and measured radioactivity. The results indicate that there is not a significant linkage between NMDA receptor activation and 'cytotoxic' edema following permanent MCAO. The detection of a large area of NMDA channel activation within regions of low ADC does however indicate an area of 'penumbral' ischemia susceptible to treatment with NMDA channel blockers.

High-resolution functional magnetic resonance imaging of the animal brain

To fully understand brain function, one must look beyond the level of a single neuron. By elucidating the spatial properties of the columnar and laminar functional architectures, information regarding the neural processing in the brain can be gained. To map these fine functional structures noninvasively and repeatedly, functional magnetic resonance imaging (fMRI) can be employed. In this article the basic principles of fMRI are introduced, including specific hardware requirements and the equipment necessary for animal magnetic resonance research. Since fMRI measures a change in secondary hemodynamic responses induced by neural activity, it is critical to understand the principles and potential pitfalls of fMRI techniques. Thus, the underlying physics of conventional blood oxygenation, cerebral blood flow, and cerebral blood volume-based fMRI techniques are extensively discussed. Tissue-specific signal change is close to the site of neural activity, while signals from large vessels can be distant from the actual active site. Thus, methods to minimize large vessel contributions and to maximize tissue signals are described. The fundamental limitation of fMRI spatial resolution is the intrinsic hemodynamic response. Based on our high-resolution fMRI studies, the hemodynamic response is regulated at submillimeter functional domains and thus spatial resolution can be achieved to an order of 100 microm. Since hemodynamic responses are sluggish, it is difficult to obtain very high temporal resolution. By using an approach with multiple experiments with different stimulus conditions, temporal resolution can be improved on the order of 100 ms. With current fMRI technologies, submillimeter columnar- and laminar-specific specific functional images can be obtained from animal brains.

Pulsed arterial spin labeling using TurboFLASH with suppression of intravascular signal

Accurate quantification of perfusion with the ADC techniques requires the suppression of the majority of the intravascular signal. This is normally achieved with the use of diffusion gradients. The TurboFLASH sequence with its ultrashort repetition times is not readily amenable to this scheme. This report demonstrates the implementation of a modified TurboFLASH sequence for FAIR imaging. Intravascular suppression is achieved with a modified preparation period that includes a driven equilibrium Fourier transform (DEFT) combination of 90 degrees-180 degrees-90 degrees hard RF pulses subsequent to the inversion delay. These pulses rotate the perfusion-prepared magnetization into the transverse plane where it can experience the suitably placed diffusion gradients before being returned to the longitudinal direction by the second 90 degrees pulse. A value of b = 20-30 s/mm(2) was thereby found to suppress the majority of the intravascular signal. For single-slice perfusion imaging, quantification is only slightly modified. The technique can be readily extended to multislice acquisition if the evolving flow signal after the DEFT preparation is considered. An advantage of the modified preparation scheme is evident in the multislice FAIR images by the preservation of the sign of the magnetization difference.

Can dynamic susceptibility contrast magnetic resonance imaging perfusion data be analyzed using a model based on directional flow?

PURPOSE

To examine the implications of a physiological model of cerebral blood that uses the contradictory assumption that blood flow in all voxels of DSCE-MRI data sets is directional in nature. Analysis of dynamic susceptibility contrast-enhanced magnetic resonance imaging (DSCE-MRI) uses techniques based on indicator dilution theory. Underlying this approach is an assumption that blood flow through pixels of gray and white matter is entirely random in direction.

MATERIALS AND METHODS

We have used a directional flow model to estimate theoretical blood flow velocities that would be observed through normal cerebral tissues. Estimates of flow velocities from individual pixels were made by measuring the mean transit time for net flow (nMTT). Measurements of nMTT were made for each voxel by estimating the mean difference in contrast arrival time between each of the adjacent six voxels.

RESULTS

Examination of the spatial distribution of contrast arrival time from DSCE-MRI data sets in normal volunteers demonstrated clear evidence of directional flow both in large vessels and in gray and white matter. The mean velocities of blood flow in gray and white matter in 12 normal volunteers were 0.25 +/- 0.013 and 0.21 +/- 0.014 cm/second, respectively, compared to predicted values of 0.25 and 0.18 cm/second. These values give measured nMTT for a 1-mm isotropic voxel of gray and white matter of 0.45 +/- 0.12 and 0.52 +/- 0.11 seconds, respectively, compared to predicted values of 0.47 and 0.55 seconds.

CONCLUSION

A directional model of blood flow provides an alternative approach to the calculation of cerebral blood flow from (CBF) DSCE-MRI data.

FMRI for Monitoring Dynamic Changes in Tissue Oxygenation/Blood Flow: Potential Applications for Tumor Response to Carbogen Treatment

The ability to differentiate between well-oxygenated and poorly-oxygenated tumors may play an important role in selecting an optimal therapeutic regime for tumor treatment of the individual patient. We present preliminary results in the development of a dynamic functional MRI method for mapping tissue oxygenation and blood flow distribution in humans simultaneously. We applied interleaved Blood Oxygenation Level Dependent (BOLD) and Flow-sensitive Alternating Inversion Recovery (FAIR) sequences to detect signals as a subject is inspiring gases of varying oxygen concentration. The method allows quantitation of the spatial distribution and time course of the important physiological functions that are easily registered with high resolution anatomic MR images. It may be used to critically evaluate the efficacy of varying durations of carbogen breathing in tumor patients, and allow a quantitative evaluation of the roles of carbogen and other radiosensitizers as potential adjuncts to radiotherapy and drug therapies.

A data post-processing protocol for dynamic MRI data to discriminate brain activity from global physiological effects

All fMRI techniques measure stimulus induced focal metabolic and physiological changes in activated brain areas. During the entire fMRI experiment it is necessary to maintain the general physiological condition of the subject as stable as possible. This is not always an easy task. The typical block design in standard fMRI experiments minimizes most of the problems related with these general physiological changes. However in some fMRI experiments, like pharmacological MRI, the experimental setup makes the use of a blocked design impossible. Therefore signal correction algorithms have been developed to correct for these physiological signal instabilities. These algorithms often require elaborate calculation efforts and the data interpretation is often very difficult if no prior knowledge on the nature of the changes exists. In this work we present an algorithm, which has the advantage of being low in calculation effort and the resulting data after correction are easy to interpret. It makes use of a datafit between the general physiological and focal activation related signal changes to eliminate the generalized effects. This algorithm has been tested on simulated and experimentally obtained signal traces suffering both from substantial general signal changes overwhelming the smaller focal activation induced signal changes.

Improved subtraction by adiabatic FAIR perfusion imaging

For pulsed arterial spin labeling techniques (e.g., FAIR), mismatches between the imaging and inversion slice profile result in a nonperfusion-related offset. Several methods have been proposed to reduce subtraction errors in FAIR imaging. Here an acquisition method for FAIR experiments based on adiabatic principles is proposed. It is shown that with adiabatic pulses the same pulse can be used for labeling and echo refocusing, thereby reducing the mismatch between imaging and labeling slice. A twofold reduction in subtraction errors compared to 5-lobe sinc excitation was shown both experimentally and by simulation.

Visual cortex reactivity in sedated children examined with perfusion MRI (FAIR)

Sleeping and sedated children can respond to visual stimulation with a decrease in blood oxygenation level dependent (BOLD) functional MRI signal response. The contribution of metabolic and hemodynamic parameters to this inverse signal response is incompletely understood. It has been hypothesized that it is caused by a relatively greater increase of oxygen consumption compared to rCBF (regional cerebral blood flow) increase. We studied the rCBF changes during visual stimulation in four sedated children, aged 4-71 months, and four alert adults, with an arterial water spin labeling technique (FAIR) and BOLD fMRI in a 1.5T MR scanner. In the children, FAIR signal decreased by a mean of 0.96% (range 0.77-1.05) of the baseline periods of the non-selective images, while BOLD signal decreased by 2.03% (range 1.99-2.93). In the adults, FAIR and BOLD signal increased by 0.88% (range 0.8-0.99) and 2.63% (range 1.99-2.93), respectively. Thus, in the children, an rCBF increase could not be detected by perfusion MRI, but indications of a FAIR signal decrease were found. An rCBF decrease in the primary visual cortex during stimulation has not been reported previously, but it is a possible explanation for the negative BOLD response. Future studies will have to address if this response pattern is a consequence of age or sleep/sedation.

Magnetic resonance studies of brain function and neurochemistry

In the short time since its introduction, magnetic resonance imaging (MRI) has rapidly evolved to become an indispensable tool for clinical diagnosis and biomedical research. Recently, this methodology has been successfully used for the acquisition of functional, physiological, and biochemical information in intact systems, particularly in the human body. The ability to map areas of altered neuronal activity in the brain, often referred to as functional magnetic resonance imaging (fMRI), is probably one of the most significant recent achievements that rely on this methodology. This development has permitted the examination of functional specialization in human and animal brains with unprecedented spatial resolution, as demonstrated by mapping at the level of orientation and ocular dominance columns in the visual cortex. These functional imaging studies are complemented by the ability to study neurochemistry using magnetic resonance spectroscopy, allowing the determination of metabolic processes that support neurotransmission and neurotransmission rates themselves.

Changes of cerebral blood flow, oxygenation, and oxidative metabolism during graded motor activation

In the present studies fMRI and a hypercapnic calibration procedure were used to monitor simultaneous changes in cerebral blood flow (CBF), cerebral blood oxygenation, and cerebral metabolic rate of oxygen (CMRO(2)) during activation in the sensorimotor cortex. In the first set of experiments seven volunteers performed bilateral, self-paced finger tapping and in the second set of experiments six volunteers performed bilateral finger tapping with six different frequencies (0.5-3 Hz). During the latter task relative CBF and BOLD signal intensity changes varied linearly as a function of stimulus frequency. In good agreement with recent PET and fMRI data increases in CMRO(2) were smaller than the corresponding changes in CBF during self-paced finger tapping and at all levels of graded motor activation. At a single level of activation and during graded activation there was a positive linear relationship between CBF and CMRO(2) with ratios of approximately 3:1. Comparable proportionality constants have been found in the visual cortex and primary sensory cortex, indicating similarities between the relationship of CBF and CMRO(2) in various cortical regions.

Comparison of the experimental BOLD signal change in event-related fMRI with the balloon model

Event-related functional MRI (ER-fMRI) based on both blood oxygenation level-dependent (BOLD) contrast and cerebral blood flow (CBF) contrast has been developed to study the human brain response to transient stimulation. A balloon model has been developed and used in understanding the relationship between changes in CBF and in the BOLD signal in block-designed fMRI. In this report, the balloon model was used to interpret experimental event-related fMRI. Using experimental data acquired from CBF-based ER-fMRI as an input function in the balloon model, simulated BOLD signal changes were obtained. The simulated time course of the BOLD signal changes was directly compared with the experimentally acquired data. A chi-square (chi(2)) test showed excellent correlation between the simulated and experimental data (alpha <or= 0.05). Our results have demonstrated that the balloon model is useful in interpreting the transient features of the experimental data from the ER-fMRI studies.

Characterization of BOLD activation in multi-echo fMRI data using fuzzy cluster analysis and a comparison with quantitative modeling

A combination of multiple gradient-echo imaging and exploratory data analysis (EDA), i.e. fuzzy cluster analysis (FCA), is proposed for separation and characterization of BOLD activation in single-shot spiral functional magnetic resonance imaging (fMRI) experiments at 3 T. Differentiation of functional activation using FCA is performed by clustering pixel signal changes (DeltaS) as a function of echo time (TE). Further vascular classification is supported by the localization of activation and the comparison with a single-exponential decay model. In some subjects, an additional indication for large vessels within a voxel was found as oscillation of the fMRI signal difference vs echo time (TE). Such large vessels may be separated from small vessel activation and, therefore, our proposed procedure might prove useful if a more specific functional localization is desired in fMRI. In addition to the signal change DeltaS, DeltaT(2)*/T(2)* is significantly different between activated regions. Averaged over all eight subjects DeltaT(2)* is 1.7 +/- 0.2 ms in ROIs with the highest signal change characterized as containing large vessels, whereas in ROIs corresponding to microvascular environment average DeltaT(2)* values are 0.8 +/- 0.1 ms.

Localized cerebral blood flow response at submillimeter columnar resolution

Functional magnetic resonance imaging (fMRI) has been widely used for imaging brain functions. However, the extent of the fMRI hemodynamic response around the active sites, at submillimeter resolution, remains poorly understood and controversial. With the use of perfusion-based fMRI, we evaluated the hemodynamic response in the cat visual cortex after orientation-specific stimuli. Activation maps obtained by using cerebral blood flow fMRI measurements were predominantly devoid of large draining vein contamination and reproducible at columnar resolution. Stimulus-specific cerebral blood flow responses were spatially localized to individual cortical columns, and columnar layouts were resolved. The periodic spacing of orientation columnar structures was estimated to be 1.1 +/- 0.2 mm (n = 14 orientations, five animals), consistent with previous findings. The estimated cerebral blood flow response at full width at half-maximum was 470 microm under single-stimulus conditions without differential subtraction. These results suggest that hemodynamic-based fMRI can indeed be used to map individual functional columns if large-vessel contributions can be minimized or eliminated.

Theoretical analysis of the effect of imperfect slice profiles on tagging schemes for pulsed arterial spin labeling MRI

Pulsed arterial spin labeling (ASL) techniques provide a noninvasive method of obtaining qualitative and quantitative perfusion images with MRI. ASL techniques employ inversion recovery and/or saturation recovery to induce perfusion weighting, and thus the performance of these techniques is dependent on the slice profiles of the inversion or saturation pulses. This article systematically examines through simulations the effects of slice profile imperfections on the perfusion signal for nine labeling schemes, including FAIR, FAIRER, and EST (UNFAIR). Each sequence is evaluated for quantitative accuracy, suppression of stationary signal, and magnitude of perfusion signal. Perfusion effects are modeled from a modified Bloch equation and experimentally determined slice profiles. The results show that FAIR, FAIRER, and EST have excellent tissue suppression. The magnitude of the perfusion signal is comparable for FAIR and FAIRER, with EST providing a slightly weaker signal. For quantitative measurements, all three methods underestimate the perfusion signal by more than 20%. Of the additional six ASL techniques examined, only one performed well in this model. This method, which combines inversion and saturation recovery, yields improved signal accuracy (<15% difference from the theoretical value) and tissue suppression similar to that of FAIR and its variants, but has only half the signal. Magn Reson Med 46:141-148, 2001.

Perfusion imaging using spin-labeling methods: contrast-to-noise comparison in functional MRI applications

In this study the performance of FLASH imaging with selective inversion preparation for functional perfusion studies was investigated. In addition to the absolute quantification of perfusion by measurement of the longitudinal relaxation times with global (T(1glob)) and selective (T(1sel)) inversion, the measurement of absolute (BASE) and relative (FAIR) perfusion increases by subtraction of appropriately weighted images was also considered. The subject averages of absolute perfusion obtained by the quantitative method were 70.7 +/- 4.0 ml/100g/min in gray matter, 10.2 +/- 3.4 ml/100g/min in white matter, and 89.0 +/- 3.1 ml/100g/min in visual cortex. These values, as well as the average increase of perfusion due to visual stimulation (44.4 +/- 3.7 ml/100g/min), agree well with respective data reported by PET and other MRI studies. However, for individual subjects the standard deviations over single ROIs inside the visual cortex lay around 100% which prevented the detection of significant activation. BASE and FAIR, on the other hand, were able to detect significant activation in single subjects. The measured average perfusion increases were 51.7 +/- 6.6 ml/100g/min and 56.5 +/- 13.8%, respectively. Magn Reson Med 46:172-182, 2001.

Separation and quantification of perfusion and BOLD effects by simultaneous acquisition of functional I(0)- and T2(*)-parameter maps

The nature of the coupling between neuronal activity and the hemodynamic response is the subject of intensive research. As a means to simultaneously measure parametric changes of T2(*), initial intensity (I(0)) and perfusion with high temporal resolution, a multi-image EPI technique with slice-selective inversion recovery (ssIR) for arterial spin labeling was developed and implemented. Comparative measurements with and without the preceding slice-selective inversion pulse were performed. I(0) and R2(*) changes induced by primary visual stimulation were separated. For ssIR-multi-image EPI the average change of I(0) over all 12 subjects was 3.4%, corresponding to a perfusion change of 40 ml/min/100 g, whereas only minor I(0) changes were observed without inversion. On average, the R2(*) of the activated pixels changed by -0.62 sec(-1) without inversion, while a significantly reduced average R2(*) change of -0.46 sec(-1) was calculated for ssIR-multi-image EPI due to a decreased BOLD effect contribution of the intravascular compartment.

Assessment of cerebrovascular reactivity with functional magnetic resonance imaging: comparison of CO(2) and breath holding

Cerebral blood flow (CBF) and oxygenation changes following both a simple breath holding test (BHT) and a CO(2) challenge can be detected with functional magnetic resonance imaging techniques. The BHT has the advantage of not requiring a source of CO(2) and acetazolamide and therefore it can easily be performed during a routine MR examination. In this study we compared global hemodynamic changes induced by breath holding and CO(2) inhalation with blood oxygenation level dependent (BOLD) and CBF sensitized fMRI techniques. During each vascular challenge BOLD and CBF signals were determined simultaneously with a combined BOLD and flow-sensitive alternating inversion recovery (FAIR) pulse sequence. There was a good correlation between the global BOLD signal intensity changes during breath holding and CO(2) inhalation supporting the notion that the BHT is equivalent to CO(2) inhalation in evaluating the hemodynamic reserve capacity with BOLD fMRI. In contrast, there was no correlation between relative CBF changes during both vascular challenges, which was probably due to the reduced temporal resolution of the combined BOLD and FAIR pulse sequence.

Changes in baseline cerebral blood flow in humans do not influence regional cerebral blood flow response to photic stimulation

The effect of changes in baseline regional cerebral blood flow (rCBF) values on the cerebral blood flow response during neuronal activation was studied with functional magnetic resonance imaging (fMRI). Using a breath-holding challenge as a hypercapnic stimulus, rCBF alterations during photic stimulation under normo- and hypercapnia were determined in nine volunteers. With breath-holding, baseline rCBF in areas corresponding to the visual cortex significantly increased from 54 +/- 5 ml/100 g/min to 85 +/- 9 ml/100 g/min (P < 0.001). Despite this significant change in baseline flow values, the rCBF increase during visual stimulation was very similar under normo- and hypercapnic conditions (28 +/- 8 ml/100 g/min versus 26 +/- 8 ml/100 g/min, respectively). This study supports the notion that within wide physiologic variations, task-induced cerebral blood flow changes are independent of baseline rCBF values.

Turbo ASL: arterial spin labeling with higher SNR and temporal resolution

A modified pulsed arterial spin labeling (ASL) technique is introduced here that has both higher temporal resolution and higher SNR per unit time than existing ASL techniques. In this technique, the time TI between the application of the tag and image acquisition is longer than the repetition time TR, allowing for the use of greatly reduced TR values without a significant decrease in the amplitude of the ASL signal. This improves both the temporal resolution and the sensitivity of ASL for functional brain mapping.

Quantitative comparison of functional contrast from BOLD-weighted spin-echo and gradient-echo echoplanar imaging at 1.5 Tesla and H2 15O PET in the whole brain

Spin-echo and gradient-echo echoplanar functional magnetic resonance imaging (fMRI) studies at 1.5 Tesla (T) were used to obtain blood oxygenation level-dependent (BOLD) contrast images of the whole brain in seven strongly right-handed women during execution of a complex motor task. Five subjects underwent subsequent H215O positron emission tomography (PET) studies while performing the same task. Group-averaged results for changes in the MRI relaxation rates R2* and R2 at 1.5T in response to neuronal activation in nine cortical, subcortical, and cerebellar motor regions are reported. Results for each method are grouped according to tissue type-cerebral cortex (precentral gyrus and supplementary motor area), subcortical regions (thalamus and putamen), and cerebellar cortex (superior lobule). The observed changes in R2* from activation-induced oxygenation changes were more variable across brain regions with different tissue characteristics than observed changes in R2. The ratio of deltaR2* to deltaR2 was 3.3 +/- 0.9 for cerebral cortex and 2.0 +/- 0.6 for subcortical tissue. deltaR2*, deltaR2, and relative blood flow changes were deltaR2* = -0.201 +/- 0.040 (s-1), deltaR2 = -0.064 +/- 0.011 s(-1), and deltaf/f = 16.7 +/- 0.8% in the cerebral cortex; deltaR2* = -0.100 +/- 0.026 s(-1), deltaR2 = -0.049 +/- 0.009 s(-1), and deltaf/f = 9.4 +/- 0.7% in the subcortical regions; and deltaR2* = -0.215 +/- 0.093 s(-1), deltaR2 = -0.069 +/- 0.012 s(-1), and deltaf/f = 16.2 +/- 1.2% in the cerebellar cortex.

Effects of hypoglycemia on functional magnetic resonance imaging response to median nerve stimulation in the rat brain

The authors studied the effects of a standardized mild-moderate hypoglycemic stimulus (glucose clamp) on brain functional magnetic resonance imaging (fMRI) responses to median nerve stimulation in anesthetized rats. In the baseline period (plasma glucose 6.6 +/- 0.3 mmol/L), the MR signal changes induced by median nerve activation were determined within a fixed region of the somatosensory cortex from preinfusion activation maps. Subsequently, insulin and a variable glucose infusion were administered to decrease plasma glucose. The goal was to produce a stable hypoglycemic plateau (2.8 +/- 0.2 mmol/L) for 30 minutes. Thereafter, plasma glucose was restored to euglycemic levels (6.0 +/- 0.3 mmol/L). In the early phase of insulin infusion (15 to 30 minutes), before hypoglycemia was reached (4.7 +/- 0.3 mmol/L), the activation signal was unchanged. However, once the hypoglycemic plateau was achieved, the activation signal was significantly decreased to 57 +/- 6% of the preinfusion value. Control regions in the brain that were not activated showed no significant changes in MR signal intensity. Upon return to euglycemia, the activation signal change increased to within 10% of the original level. No significant activation changes were noted during euglycemic hyperinsulinemic clamp experiments. The authors concluded that fMRI can detect alterations in cerebral function because of insulin-induced hypoglycemia. The signal changes observed in fMRI activation experiments were sensitive to blood glucose levels and might reflect increases in brain metabolism that are limited by substrate deprivation during hypoglycemia.

Comparison of simultaneously measured perfusion and BOLD signal increases during brain activation with T(1)-based tissue identification

Perfusion and blood oxygenation level-dependent (BOLD) signals were simultaneously measured during a finger-tapping task at 3T using QUIPSS II with thin-slice TI(1) periodic saturation, a modified pulsed arterial spin labeling technique that provides quantitative measurement of perfusion. Perfusion and BOLD signal changes due to motor activation were obtained and correlated with the T(1) values estimated from echo-planar imaging (EPI)-based T(1) maps on a voxel-by-voxel basis. The peak perfusion signal occurs in voxels with a T(1) of brain parenchyma while the peak BOLD signal occurs in voxels with a T(1) characteristic of blood and cerebrospinal fluid. The locations of the peak signals of functional BOLD and perfusion only partially overlap on the order of 40%. Perfusion activation maps will likely represent the sites of neuronal activity better than do BOLD activation maps. Magn Reson Med 44:137-143, 2000.

Quantitation of renal perfusion using arterial spin labeling with FAIR-UFLARE

Quantitative perfusion imaging of human kidneys was performed using arterial spin labeling MRI with a fast spin echo readout-sequence. Perfusion maps of centrally located single slices were obtained in axial and coronal orientations. In ten healthy volunteers, the mean value of perfusion was 213+/-55 mL/(100g min) with a range from 140 to 319 mL/(100g min). These results are in accordance with literature data, considering the fact that FAIR only measures the perfusion component normal to the imaging plane. Intra-individual reproducibility errors of +/-11% were smaller than the natural interindividual variability of renal perfusion (SD = +/- 25%). Perfusion in the cortex was approximately 3-4 times higher compared to the medulla. Considering the relatively high resolution of 2x2x10 mm3, the ability to quantify perfusion, and the lack of ionizing radiation and contrast media, this technique should prove useful in diagnosing renal pathologies that are associated with reductions in tissue perfusion.